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Advanced High Speed Overrunning Clutch for Rotorcraft Transmissions

Description:

OBJECTIVE: Develop and demonstrate a novel, high speed overrunning clutch concept with high power density, low manufacturing costs and excellent durability and reliability. DESCRIPTION: Overrunning clutches are a critical component of the drive train of all helicopters. The overrunning clutch prevents the flow of torque from the thrust producing rotors back into the engine(s) during autorotation and high energy maneuvers, and allows the individual start-up and shut down of engines on multi-engine aircraft. Conventional overrunning clutch designs rely on the friction of wedged shaped elements in the form of sprags or ramps and rollers to allow torque to be transferred in only one direction of rotation. The overrunning clutch is sometimes called a mechanical diode referring to its electrical analog. Other concepts utilizing expanding springs and helical splines with centrifugally actuated engagement mechanisms have been pursued over the last 20 years with limited success. The primary failure mode is failure to engage as a result of excessive wear of sliding surfaces accumulated during overrunning. Current overrunning clutches are located after the first geared reduction stage and thus operate at speeds between 8,000 and 6,000 rpm. Since weight is a direct function of transmitted torque, and torque is a direct function of speed, it is most weight efficient to locate and operate the clutch at the highest speed in the drive train. Typical turboshaft engine output speeds are 15,000 rpm for engines of 4000 hp or more and 20,000 rpm for engines up to 3000 hp. Achieving a long life in an overrunning clutch operating at these high speeds is challenging. This topic seeks to develop an innovative overrunning clutch design which minimizes the wear and heat generation associated with full and partial speed overrunning of clutches operating at high speeds. Overrunning clutches, as used in the main drive trains of multi engine rotorcraft operate in only three modes: locked, differential overrunning (input speed 50% to 70% of output), and full-speed overrunning (input speed zero and output speed 100%). The locked position is the most common occurring all times when the engine is driving the transmission. Differential speed overrunning, which is by far the worst condition for wear, occurs mainly during practice autorotations with both engines set to ground idle, and the main rotor over speeding up to 105% rpm. Full speed overrunning occurs mainly during engine start up and in flight with one engine inoperative (OEI). For a medium size twin engine utility rotorcraft without a rotorbrake, a typical lifetime is 12,400 flight hours. During this lifetime, 124 hrs will be spent in differential overrunning mode, and 426 hrs will be spent in full-sped overrunning mode with 12 of these hrs representing OEI operations. Modern sprag or ramp roller clutch designs (UH-60 and AH-64) are typicaly located after the first stage of reduction gearing and thus operate at speeds around 8,000 rpm with a static rating of 3000 hp. Given the design lives stated above, these clutches weigh approximately 20.00 lbs. This results in a power density of 150 hp/lb. This topic seeks to develop an overrunning clutch which can successfully operate at 20,000 rpm input speed (lower torque position) with a 30% increase in power density and no reduction in durability or reliability and the potential for reduced production cost. PHASE I: During Phase I effort, the contractor should conduct a conceptual/preliminary design to analytically determine the potential performance, weight, durability and cost attributes of the proposed overrunning clutch design. The contractor should utilize a current rotorcraft manufacturer as a consultant for this effort. The results of this activity shall be presented to the Government for review. Technical challenges associated with key features of the proposed design shall be studied in more detail. Small scale coupon or bench testing may be conducted to to gather data regarding these key challenges and the merit of potential approaches towards their solution. Key performance metrics to be achieved in the design include operation at 20,000 rpm input speed, and 3000 hp, 30% increase in power density (195 hp/lb), reduced production cost and reliability equal to or greater than current sprag and ramp roller designs. PHASE II: During Phase II, the contractor should utilize the results of the phase I effort and conduct a detailed design and analysis of the light weight high speed overrunning clutch design. The detailed design effort should fully evaluate the performance and durability of the proposed clutch in all of the critical operating conditions. The contractor should utilize a current rotorcraft manufacturer as a consultant for this effort. The results of the detailed design should be presented to the government prior to the initiation of hardware fabrication. The contractor should then proceed with fabrication of full scale hardware for use in validation testing. The clutch design should be tested for engagement/disengagement and full and partial speed overrunning at the full design speed of 20,000 rpm. Static testing may be utilized to validate the ability of the clutch to engage and transmit the full design torque. A TRL of 5 is the desired end-state of the Phase II effort. As in Phase I, key performance metrics include operation at 20,000 rpm input speed and 3000 hp, 30% increase in power density (195 hp/lb), reduced production cost and reliability equal to or greater than current sprag and ramp roller designs. PHASE III: The Phase III program is envisioned as an effort where the advanced high speed overrunning clutch technology developed in the previous phases has proven to be attractive and has garnered the attention of a rotorcraft manufacturer to the point where they are willing to invest in it further. Additional effort under Phase III would include minor modifications to the design to further enhance producibility and reduce cost. Additional testing utilizing a rig that is closely representative of the actual aircraft installation would be conducted to fully evaluate the clutch durability. Upon successful completion of this testing the clutch would be assembled into the actual aircraft drive train and subjected to ground based testing followed by flight testing. Potential military applications include future upgrades to the Sikorsky uh-60, Bell-Boeing V-22, Boeing CH-47 and AH-64, and the Bell OH-58. The Army is also planning for the development of a new medium class of helicopters known as the Future Vertical Lift. This could be a future application of the clutch. The drive train requirements and designs of commercial and military rotorcraft differ very little. Thus the advance high speed overrunning clutch would have a strong transition path to commercial rotorcraft such as the Sikorsky S-76 and S-92, and the Bell 407 and 429. REFERENCES: 1. Advanced Overrunning Clutch Technology, USAAMRDL-TR-77-16, Jules G. Kish, Sikorsky Aircraft, December 1977, Approved for Public release, Distribution unlimited 2. PositiveEngagement Clutch, USAAMRDL-TR-73-96, Charles J. Wirth, Kaman Aerospace Corporation, December 1973, Approved for Public release, Distribution unlimited 3. Sprag Overriding Aircraft Clutch, USAAMRDL-TR-72-49, P. Lyander, AVCO Lycoming, July 1972, Approved for Public release, Distribution unlimited 4. Helicopter Freewheel Unit design Guide, USAAMRDL-TR-77-18, Jules Kish, Sikorsky Aircraft, October 1977, Approved for Public release, Distribution unlimited 5. Design, Fabrication and Testing of a High-Speed, Over-Running Clutch for Rotorcraft, NASA CR-1998-208513 ARL-CR-429, August 1998, F. Fitz and C. Gadd, Approved for Public release, Distribution unlimited
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